1. Field of the Invention
This invention relates to novel waterdispersible cationic resins with a specific composition and to their use in the cathodic electrical covering of electrically conducting objects.
2. Brief Description of the Background of the Invention Including Prior Art
For some time the cathodic electrical covering has been performed on an industrial scale. Its principle is to submerge an electrically conducting object into an aqueous dispersion of a cationic polymeric material. The current fed from the electrically conducting and as a cathode acting object through the dispersion to the anode as a counterelectrode effects the deposition at the cathode.
It is disclosed in German Offenlegungsschrift DT-OS No. 2,252,536 to prepare a waterdispersible cationic resin by reacting an organic synthetic resin with epoxy groups with a primary or secondary amine and by reacting the resulting product with a half-masked organic polyisocyanate, wherein this polyisocyanate comprises on the average one free isocyanate group and wherein at room temperature the masked isocyanate groups are stable against said product, but are reacting at elevated temperatures with hydroxy groups.
It has been proposed in German Offenlegungsschrift DT-OS No. 2,541,234 to produce a cationic electrophoretic layer composition by contacting a partially masked polyisocyanate compound with a defined starting material. The starting material is synthesized by reacting an epoxy resin with an aliphatic or alicyclical amino compound, followed by admixing to the resulting reaction product either a polyamide resin or by reacting the resulting reaction product with a polyamide resin.
The conventional resins for electrodeposition at a cathode do not meet their purpose in an optimal way, since the elasticity of the film and the stone impact strength are not fully satisfactory. In addition, in their case also the so-called pin hole formation occurs, which can be recognized under the microscope by hole like recesses and which exerts a negative influence on the corrosion stability. A further weak point is the adhesion of the cataphoretic lacquer to the intermediate lacquer, which requires improvement.